A method for forming a wafer level package that is equipped with solder balls on a top surface and encapsulated by a stress buffer layer of an elastomeric material is disclosed. The method can be carried by first forming a plurality of solder balls on bond pads provided on a top surface of a wafer and then forming an elastomeric material layer, or any other flexible, compliant material layer to encapsulate the solder balls. The tip portions of the solder balls is then substantially exposed by an etching process of either dry etching or wet etching such that the solder balls can be connected electrically to a circuit board. The present invention further provides a wafer level package that is formed with solder balls on a top surface encapsulated in an elastomeric material layer. The elastomeric material layer serves both as a stress buffer and a thermal expansion buffer such that the integrity and reliability of IC devices severed from the wafer can be maintained.

The present invention discloses a dual-photoresist method for forming fine-pitched solder bumps on flip chips by utilizing two separate layers of photoresist, i.e., a first thin photoresist layer for patterning the BLM layers on top of the aluminum bonding pads and a second thick photoresist layer for patterning the via openings on top of the BLM layers to supply the necessary thickness required for the solder bumps. The first, thin photoresist layer permits an accurate imaging process to be conducted without focusing problems which are normally associated with thick photoresist layers. As an optional step, the present invention may further utilize a thin layer of non-leachable metal such as Cu or Ni for coating on top of the BLM layer and thus further improving the electrical characteristics of the solder bumps subsequently formed thereon. A majority of the BLM layer is removed with the first, thin photoresist layer and thus, in the final BLM removal process, only a very thin adhesion sublayer of the BLM layer needs to be removed and as a result, ensures a clean removal process without damaging the solder bumps already formed with a fine-pitch.